Nickel(II)/Lewis acid catalysed olefin hydroamination and hydroarylation under mild conditions.

Aniline derivatives are important nitrogen-containing compounds with wide applications in chemicals, pharmaceuticals and agrochemicals. In the work described herein, nickel(II)/Lewis acid (LA) catalysed olefin hydroamination with anilines was explored for use in aniline derivative syntheses. The Ni(II)/LA catalysis proceeded smoothly under mild conditions, whereas using Ni(OAc)2 alone, the catalyst was inactive. Remarkably, the Markovnikov addition type products were obtained when substituted styrenes were used as the olefin source, while the anti-Markovnikov addition type products were obtained when the electron-deficient olefins such as acrylonitrile and acrylates were used. The mechanistic studies revealed that hydroamination of the styrene derivates proceeded via the amino-Ni(II)/LA attacking the carbocation intermediate which was generated by the protonation of the olefin, whereas for acrylonitrile and acrylates, it proceeded by a direct amino-Ni(II)/LA attack on the olefin by nucleophilic addition. In addition, the hydroarylation product was generated by the Hofmann-Martius rearrangement of the hydroamination product.

Pd(II)/LA-catalyzed acetanilide olefination with dioxygen.

Transition-metal-catalyzed aromatic olefination through direct C-H activation represents an atom and step-economic route for versatile pharmaceutical syntheses, and in many cases, different stoichiometric oxidants are frequently employed for achieving a reasonable catalytic efficiency of the transition metal ions. Herein, we report a Lewis acid promoted Pd(II)-catalyzed acetanilide olefination reaction with atmospheric dioxygen as the oxidant source. The linkage of the Lewis acid to the Pd(II) species through a diacetate bridge significantly improved its catalytic efficiency, and independent kinetic studies on the olefination step revealed that adding the Lewis acid significantly accelerated the olefination rate as well as the C-H activation step. A strong basicity of the internal base in the Pd(II) salt also benefited the olefination reaction plausibly through base-assisted ß-hydride elimination.

Observing the Agostic Hydrogen in Pd(II)-Catalyzed Aromatic C-H Activation.

Direct C-H activation and functionalization offer a convenient protocol for pharmaceutical and material syntheses. Although versatile mechanisms have been proposed to depict transition-metal-catalyzed C-H activation, to date, the shared key agostic hydrogen intermediate in several major mechanisms has not been observed yet, which apparently puzzles the mechanism-based catalyst design. This work reports the direct observations of this intermediate in Pd(II)/Sc(III)-catalyzed C-H activation of acetanilides, and its stability and reactivity in C-H activation are investigated. Remarkably, this intermediate is only observed in electron-rich acetanilides, and the meta-substituent with increased σm constant generally accelerates C-H activation, a characteristic of the base-assisted C-H activation mechanism. This study has unveiled the masks of this intermediate with an understanding of its first-hand physicochemical properties, shedding new light on mechanism-based catalyst design.

Pd(II)/Lewis Acid Catalyzed Intramolecular Annulation of Indolecarboxamides with Dioxygen through Dual C-H Activation.

Transition-metal ion catalyzed intramolecular dual C-H activation to construct polycyclic heteroarene skeletons is merited for its step and atom-economic advantages in organic synthesis. However, in most cases, stoichiometric oxidants, elevated temperature, and other harsh conditions were commonly faced for this reaction, which apparently block the synthetic applications. Herein, we report a Pd(II)/LA (LA: Lewis acid) catalyzed intramolecular dual C-H activation to construct indoloquinolinone derivatives under mild conditions with dioxygen as the sole oxidant. It was found that adding LA such as Sc3+ to Pd(OAc)2 sharply improved its catalytic efficiency, whereas Pd(OAc)2 alone was very sluggish. The activity improvement was attributed to the linkage of the Sc3+ cation to the Pd(II) species through a diacetate bridge that significantly enhanced the electrophilic properties of Pd(II) for dual C-H activation.

Palladium(II)/Lewis Acid-Catalyzed Olefination of Arylacetamides with Dioxygen.

The present work introduces Pd(II)/LA-catalyzed (LA: Lewis acid) olefination of arylacetamides with dioxygen as the oxidant source. This protocol tolerates with different functional groups on the substrates, and the catalytic efficiency is highly Lewis acidity-dependent on added LA, that is, a stronger LA provided a better promotional effect. The 1H NMR studies of the semireaction between the arylacetamide and the Pd(II)/Sc(III) catalyst in HOAc-d4 disclosed the formation of a palladacycle intermediate, and the C-H activation step was reversible, which led to the formation of the deuterated arylacetamide substrate and the palladacycle intermediate. Further semireaction between the palladacycle intermediate and the olefin disclosed that it was a clean and much faster reaction than the C-H activation step, thus revealing multiple mechanistic information for Pd(II)-catalyzed C-H activation.

Pd(II)/Lewis acid catalyzed regioselective olefination of indole with dioxygen.

Transition metal ion catalyzed indole olefination through C-H activation is a convenient protocol to synthesize versatile bioactive vinylindole compounds; however, in most cases, stoichiometric amounts of oxidants were necessary to accomplish the catalytic cycle. The present study describes a Pd(II)/LA (LA: Lewis acid) catalyzed indole olefination with dioxygen as the sole oxidant. The olefination reaction with electron-rich olefins proceeded smoothly through the pyrrolyl N-carboxamide group directed remote C-H activation at the C3 position of the indole with the Pd(II)/LA catalyst, whereas Pd(II) alone was a very sluggish catalyst under identical conditions. For the electron-deficient olefins, the directing N-carboxamide group was not essential for olefination with this Pd(II)/LA catalyst, demonstrating a different olefination pathway from that of electron-rich olefins. Remarkably, 1H NMR kinetics disclosed that olefination proceeded much faster with electron-rich olefins than with electron-deficient ones.

Configuration-Dependent Liquid Crystal and Gel Behaviors of Tetraphenylethene-Containing Main-Chain Copolyesters.

The construction of aggregation-induced emission-active (AIE-active) gelators with liquid crystal properties remains a challenge. Moreover, the effects of AIE configuration on liquid crystal, gel and AIE behaviors in one system are unclear. Herein, two main-chain liquid crystalline copolyester gelators with a single configuration of AIEgen TPE, mesogen biphenyl, and pendent amide groups are synthesized through melt polycondensation. Both copolyesters display smectic phase, while E-P20 possesses a wider temperature range of liquid crystal and a narrower layer distance owing to the more serious nonlinear "defect" of Z-TPE than E-TPE units. In addition, E-P20 and Z-P20 can form AIE-active gels with the minimum gelation concentration (MGC) values of 10 and 4 wt% in ethyl acetate mainly via hydrogen bonds between the pendent amide groups, respectively. These AIE-active gels show potential applications in temperature sensor, information storage, and so on.

Decarboxylative Addition of Propiolic Acids with Indoles to Synthesize Bis(indolyl)methane Derivatives with a Pd(II)/LA Catalyst.

Exploring new protocols for efficient organic synthesis is crucial for pharmaceutical developments. The present work introduces a Pd(II)/LA-catalyzed (LA: Lewis acid) decarboxylative addition reaction for the synthesis of bis(indolyl)methane derivatives. The presence of Lewis acid such as Sc(OTf)3 triggered Pd(II)-catalyzed decarboxylative addition of propiolic acids with indoles to offer the bis(indolyl)methane derivatives in moderate to good yields, whereas neither Pd(II) nor Lewis acid alone was active for this synthesis. The catalytic efficiency of Pd(OAc)2 was highly dependent on the Lewis acidity of the added Lewis acid, that is, a stronger Lewis acid provided a higher yield of the bis(indolyl)methane derivatives. Meanwhile, this Pd(II)/LA-catalyzed decarboxylative addition reaction showed good tolerance toward versatile electron-rich or -deficient substituents on the indole skeleton and on the benzyl ring of propiolic acids. The studies on the in situ 1H NMR kinetics of this Pd(II)/Sc(III) catalysis disclosed the formation of a transient vinyl-Pd(II)/Sc(III) intermediate generated by the pyrrole addition to the alkynyl-Pd(II)/Sc(III) species after decarboxylation, which was scarcely observed before.

Palladium(II)/Lewis Acid-Catalyzed Oxidative Olefination/Annulation of N-Methoxybenzamides: Identifying the Active Intermediates through NMR Characterizations.

Although Pd(II)-catalyzed C-H activation in arenes has been widely successful in organic synthesis with many palladacycle compounds isolated as the intermediates in ligand-directed C-H activation, direct identification of the reaction intermediates such as the π-complex prior to the C-H activation is still not successful because of their instability. In the present study, we introduce a Pd(II)/LA (LA: Lewis acid)-catalyzed oxidative olefination/annulation reaction between N-methoxybenzamides and acrylates with oxygen as the oxidant source, in which two intermediates, including an unsymmetrical η6-complex and a palladacycle species without the proton releasing to the environment, were identified through NMR characterizations. The in situ formation of the heterobimetallic Pd(II)/LA species such as Pd(II)/Sc(III) may have enhanced the electrophilic properties of the Pd2+ cation, thus improving the stability of the π-complex, herein, an unsymmetrical η6-complex, and improving its catalytic efficiency. The observed insensitive electronic effect preferred the concerted metalation-deprotonation (CMD) mechanism for this C-H activation, and the detected palladacycle intermediate without the proton releasing to the environment offered an experimental clue to support the proposed CMD mechanism.

Lewis Acid Promoted Aerobic Oxidative Coupling of Thiols with Phosphonates by Simple Nickel(II) Catalyst: Substrate Scope and Mechanistic Studies.

Exploring new catalysts for efficient organic synthesis is among the most attractive topics in chemistry. Here, using Ni(OAc)2/LA as catalyst (LA: Lewis acid), a novel catalyst strategy was developed for oxidative coupling of thiols and phosphonates to phosphorothioates with oxygen oxidant. The present study discloses that when Ni(OAc)2 alone was employed as the catalyst, the reaction proceeded very sluggishly with low yield, whereas adding non-redox-active metal ions such as Y3+ to Ni(OAc)2 dramatically promoted its catalytic efficiency. The promotional effect is highly Lewis acidity dependent on the added Lewis acid, and generally, a stronger Lewis acid provided a better promotional effect. The stopped-flow kinetics confirmed that adding Y(OTf)3 can obviously accelerate the activation of thiols by Ni(II) and next accelerate its reaction with phosphonate to generate the phosphorothioate product. ESI-MS characterizations of the catalyst disclosed the formation of the heterobimetallic Ni(II)/Y(III) species in the catalyst solution. Additionally, this Ni(II)/LA catalyst can be applied in the synthesis of a series of phosphorothioate compounds including several commercial bioactive compounds. This catalyst strategy has clearly supported that Lewis acid can significantly improve the catalytic efficiency of these traditional metal ions in organic synthesis, thus opening up new opportunities in their catalyst design.

Nonredox Metal-Ions-Enhanced Dioxygen Activation by Oxidovanadium(IV) Complexes toward Hydrogen Atom Abstraction.

Dioxygen activation toward efficient catalysis at ambient temperature is still a big challenge for industrial oxidations, while it proceeds smoothly in nature. This work presents an example of that adding nonredox metal ions as Lewis acid can enhance dioxygen activation by oxidovanadium(IV) complex, [VIV(O)Cl(TPA)]PF6 (where TPA is tris-[(2-pyridy)methyl]amine), which leads to efficient hydrogen abstraction at ambient temperature, whereas, in the absence of a Lewis acid, the catalytic hydrogen abstraction of the oxidovanadium(IV) complex is very sluggish. Ultraviolet-visible light (UV-vis), electron paramagnetic resonance (EPR), mass, and nuclear magnetic resonance (NMR) studies have provided informative clues to indicate the interaction between the Lewis acid and vanadium complexes, including assisting the dissociation of the chloride from the oxidovanadium(IV) complex, interacting with the vanadium oxido group, and stabilizing the vanadium(V) superoxo species. These interactions enhanced the dioxgyen activation efficiency of oxidovanadium(IV) complex, and improved the hydrogen abstraction ability of vanadium(V) oxido species, which leads to efficient hydrogen abstraction in a catalytic process. A brief mechanism has also been proposed for dioxygen activation toward hydrogen abstraction by an oxidovanadium(IV) complex.

Nonredox Metal Ions Promoted Olefin Epoxidation by Iron(II) Complexes with H2O2: DFT Calculations Reveal Multiple Channels for Oxygen Transfer.

Nonredox metal ions play significant roles in a wide range of biological and chemical oxidations in which they can modulate the oxidative reactivity of those redox metal ions. With environmentally benign H2O2 as oxidant, the influence of nonredox metal ions on an iron(II) complex mediated olefin epoxidation was investigated through experimental studies and theoretical calculations. It was found that adding nonredox metal ions like Sc3+ can substantially improve the oxygen transfer efficiency of the iron(II) complex toward cyclooctene epoxidation even in the presence of certain amount of water. In 18O-labeling experiments with 18O water, the presence of Sc3+ provided a higher 18O incorporation in epoxide. In UV-vis studies, it was found that the presence of Sc3+ makes both FeIII-OOH and FeIV═O species unstable. Density functional theory calculations further disclosed that, in the presence of Sc(OTf)3, the Sc3+ adducts of FeIII-OOH and FeIV═O species are capable of epoxidizing olefin as well as FeV═O species, thus opening multiple channels for oxygenation. In particular, in the pathway of cyclooctene epoxidation, the FeIV═O/Sc3+ adduct-mediated epoxidation is more energetically favorable than that of the FeV═O species (12.2 vs 17.2 kcal/mol). This information may implicate that the presence of certain nonredox metal ions can facilitate these redox metal ions mediating biological and chemical oxidations happening at a relatively low oxidation state, which is more energetically accessible.

Non-redox metal ion promoted oxidative coupling of indoles with olefins by the palladium(ii) acetate catalyst through dioxygen activation: experimental results with DFT calculations.

Developing new catalytic technologies through C-H bond activation to synthesize versatile pharmaceuticals has attracted much attention in recent decades. This work introduces a new strategy in catalyst design for Pd(ii)-catalyzed C-H bond activation in which non-redox metal ions serving as Lewis acids play significant roles. In the oxidative coupling of indoles with olefins using dioxygen, it was found that Pd(OAc)2 alone as the catalyst is very sluggish at ambient temperature which provided a low yield of the olefination product, whereas adding non-redox metal ions to Pd(OAc)2 substantially improves its catalytic efficiency. In particular, it provided bis(indolyl)methane derivatives as the dominant product, a category of pharmacological molecules which could not be synthesized by Pd(ii)-catalyzed oxidative coupling previously. Detailed investigations revealed that the reaction proceeds by heterobimetallic Pd(ii)/Sc(iii)-catalyzed oxidative coupling of an indole with an olefin followed by Sc(iii)-catalyzed addition with a second indole molecule. DFT calculations disclosed that the formation of heterobimetallic Pd(ii)/Sc(iii) species substantially decreases the C-H bond activation energy barrier, and shifts the rate determining step from C-H bond activation of indole to the olefination step. This non-redox metal ion promoted Pd(ii)-catalyzed C-H bond activation may offer a new opportunity for catalyst design in organic synthesis, which has not been fully recognized yet.

The reactivity of the active metal oxo and hydroxo intermediates and their implications in oxidations.

While the significance of the redox metal oxo moieties has been fully acknowledged in versatile oxidation processes, active metal hydroxo moieties are gradually realized to play the key roles in certain biological oxidation events, and their reactivity has also been evidenced by related biomimic models. However, compared with the metal oxo moieties, the significance of the metal hydroxo moieties has not been fully recognized, and their relationships in oxidations remain elusive until recently. This review summarizes the reactivity of the metal oxo and hydroxo moieties in different oxidation processes including hydrogen atom transfer, oxygen atom transfer and electron transfer, and their reactivity similarities and differences have been discussed as well. Particularly, how the physicochemical properties like metal-oxygen bond order, net charge and potential of a redox metal ion affect its reactivity has also been presented based on available data. We hope that this review may provide new clues to understand the origins of the enzyme's choice on them in a specific event, to explain the elusive phenomena occurring in those enzymatic, homogeneous and heterogeneous oxidations, to design selective redox catalysts and control their reactivity.

Synthesis, structural studies, and oxidation catalysis of the late-first-row-transition-metal complexes of a 2-pyridylmethyl pendant-armed ethylene cross-bridged cyclam.

The first 2-pyridylmethyl pendant-armed ethylene cross-bridged cyclam ligand has been synthesized and successfully complexed to Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) cations. X-ray crystal structures were obtained for all six complexes and demonstrate pentadentate binding of the ligand with the requisite cis-V configuration of the cross-bridged cyclam ring in all cases, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of the complexes were evaluated using solid-state magnetic moment determination and acetonitrile solution electronic spectroscopy, which both agree with the crystal structure determination of high-spin divalent metal complexes in all cases. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all but the Ni(2+) complex, suggesting that catalytic reactivity involving electron-transfer processes is possible for complexes of this ligand. Kinetic studies of the dissociation of the ligand from the copper(II) complex under strongly acidic conditions and elevated temperatures revealed that the pyridine pendant arm actually destabilizes the complex compared to the parent cross-bridged cyclam complex. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant for the most biologically relevant Mn(2+), Fe(2+), and Cu(2+) complexes identified the Mn(2+) complex as a potential mild oxidation catalyst worthy of continued development.

Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam.

The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and successfully complexed to Mn2+, Fe2+, and Cu2+ cations. X-ray crystal structures were obtained for the diprotonated ligand and its Cu2+ complex demonstrating pentadentate binding of the ligand with trans-II configuration of the side-bridged cyclam ring, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of these complexes were determined by means of solid state magnetic moment, with a low value of µ = 3.10 µB for the Fe2+ complex suggesting it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu2+ complex, while the µ = 5.52 µB value for the Mn2+ complex suggests it is high spin octahedral. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting catalytic reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant identified the Fe2+ complex as the oxidation catalysts most worthy of continued development.

Understanding the oxidative relationships of the metal oxo, hydroxo, and hydroperoxide intermediates with manganese(IV) complexes having bridged cyclams: correlation of the physicochemical properties with reactivity.

Multiple transition metal functional groups including metaloxo, hydroxo, and hydroperoxide groups play significant roles in various biological and chemical oxidations such as electron transfer, oxygen transfer, and hydrogen abstraction. Further studies that clarify their oxidative relationships and the relationship between their reactivity and their physicochemical properties will expand our ability to predict the reactivity of the intermediate in different oxidative events. As a result researchers will be able to provide rational explanations of poorly understood oxidative phenomena and design selective oxidation catalysts. This Account summarizes results from recent studies of oxidative relationships among manganese(IV) molecules that include pairs of hydroxo/oxo ligands. Changes in the protonation state may simultaneously affect the net charge, the redox potential, the metal-oxygen bond order (M-O vs M═O), and the reactivity of the metal ion. In the manganese(IV) model system, [Mn(IV)(Me(2)EBC)(OH)(2)](PF(6))(2), the Mn(IV)-OH and Mn(IV)═O moieties have similar hydrogen abstraction capabilities, but Mn(IV)═O abstracts hydrogen at a more than 40-fold faster rate than the corresponding Mn(IV)-OH. However, after the first hydrogen abstraction, the reduction product, Mn(III)-OH(2) from the Mn(IV)-OH moiety, cannot transfer a subsequent OH group to the substrate radical. Instead the Mn(III)-OH from the Mn(IV)═O moiety reforms the OH group, generating the hydroxylated product. In the oxygenation of substrates such as triarylphosphines, the reaction with the Mn(IV)═O moiety proceeds by concerted oxygen atom transfer, but the reaction with the Mn(IV)-OH functional group proceeds by electron transfer. In addition, the manganese(IV) species with a Mn(IV)-OH group has a higher redox potential and demonstrates much more facile electron transfer than the one that has the Mn(IV)═O group. Furthermore, an increase in the net charge of the Mn(IV)-OH further accelerates its electron transfer rate. But its influence on hydrogen abstraction is minor because charge-promoted electron transfer does not enhance hydrogen abstraction remarkably. The Mn(IV)-OOH moiety with an identical coordination environment is a more powerful oxidant than the corresponding Mn(IV)-OH and Mn(IV)═O moieties in both hydrogen abstraction and oxygen atom transfer. With this full understanding of the oxidative reactivity of the Mn(IV)-OH and Mn(IV)═O moieties, we have clarified the correlation between the physicochemical properties of these active intermediates, including net charge, redox potential, and metal-oxygen bond order, and their reactivities. The reactivity differences between the metal oxo and hydroxo moieties on these manganese(IV) functional groups after the first hydrogen abstraction have provided clues for understanding their occurrence and functions in metalloenzymes. The P450 enzymes require an iron(IV) oxo form rather than an iron(IV) hydroxo form to perform substrate hydroxylation. However, the lipoxygenases use an iron(III) hydroxo group to dioxygenate unsaturated fatty acids rather than an iron(III) oxo species, a moiety that could facilitate hydroxylation reactions. These distinctly different physicochemical properties and reactivities of the metal oxo and hydroxo moieties could provide clues to understand these elusive oxidation phenomena and provide the foundation for the rational design of novel oxidation catalysts.

Influence of calcium(II) and chloride on the oxidative reactivity of a manganese(II) complex of a cross-bridged cyclen ligand.

Available data from different laboratories have confirmed that both Ca(2+) and Cl(-) are crucial for water oxidation in Photosystem II. However, their roles are still elusive. Using a manganese(II) complex having a cross-bridged cyclen ligand as a model, the influence of Ca(2+) on the oxidative reactivity of the manganese(II) complex and its corresponding manganese(IV) analogue were investigated. It has been found that adding Ca(2+) can significantly improve the oxygenation efficiency of the manganese(II) complex in sulfide oxidation and further accelerate the oxidation of sulfoxide to sulfone. Similar improvements have also been observed for Mg(2+), Sr(2+), and Ba(2+). A new monomeric manganese(IV) complex having two cis-hydroxide ligands has also been isolated through oxidation of the corresponding manganese(II) complex with H2O2 in the presence of NH4PF6. This rare cis-dihydroxomanganese(IV) species has been well characterized by X-ray crystallography, electrochemistry, electron paramagnetic resonance, and UV-vis spectroscopy. Notably, using the manganese(IV) complex as a catalyst demonstrates higher activity than the corresponding manganese(II) complex, and adding Ca(2+) further improves its catalytic efficiency. However, adding Cl(-) decreases its catalytic activity. In electrochemical studies of manganese(IV) complexes with no chloride ligand present, adding Ca(2+) positively shifted the redox potential of the Mn(IV)/Mn(III) couple but negatively shifted its Mn(V)/Mn(IV) couple. In the manganese(II) complex having a chloride ligand, adding Ca(2+) shifted both the Mn(IV)/Mn(III) and Mn(V)/Mn(IV) couples in the negative direction. The revealed oxidative reactivity and redox properties of the manganese species affected by Ca(2+) and Cl(-) may provide new clues to understanding their roles in the water oxidation process of Photosystem II.

Degradation of chlorophenols by supported Co-Mg-Al layered double hydrotalcite with bicarbonate activated hydrogen peroxide.

Toxic and bioresistant compounds have attracted researchers to develop more efficient and cost-effective technologies for degradation of organic compounds in wastewater. This work demonstrates the degradation of 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and phenol as model compounds using bicarbonate-activated H2O2 oxidation system in the presence of supported catalysts. The catalytic activity of the catalyst was investigated in term of degradation of target compounds, chemical oxygen demand (COD), and total organic carbon (TOC) removals both for batch mode and in fixed bed reactor using CoMgAl-HTs and CoMgAl-SHTs, respectively. The leaching of cobalt ion was efficiently prohibited because of the presence of a weakly basic medium provided by bicarbonate, and the CoMgAl-SHTs catalyst was found to retain its stability and good catalytic activity in fixed bed reactor for over 300 h. Extensive chemical probing, fluorescence, and electron paired resonance (EPR) studies were conducted to identify the actual reactive species in the degradation pathway, which revealed that the reaction proceeds through generation of superoxide, hydroxyl radical along with carbonate radical.

Lewis-acid-promoted stoichiometric and catalytic oxidations by manganese complexes having cross-bridged cyclam ligand: a comprehensive study.

Redox-inactive metal ions have been recognized to be able to participate in redox metal-ion-mediated biological and chemical oxidative events; however, their roles are still elusive. This work presents how the redox-inactive metal ions affect the oxidative reactivity of a well-investigated manganese(II) with its corresponding manganese(IV) complexes having cross-bridged cyclam ligand. In dry acetone, the presence of these metal ions can greatly accelerate stoichiometric oxidations of triphenylphosphine and sulfides by the manganese(IV) complexes through electron transfer or catalytic sulfoxidations by the corresponding manganese(II) complexes with PhIO. Significantly, the rate enhancements are highly Lewis-acid strength dependent on added metal ions. These metal ions like Al(3+) can also promote the thermodynamic driving force of the Mn(IV)-OH moiety to facilitate its hydrogen abstraction from ethylbenzene having a BDE(CH) value of 85 kcal/mol, while it is experimentally limited to 80 kcal/mol for Mn(IV)-OH alone. Adding Al(3+) may also improve the manganese(II)-catalyzed olefin epoxidation with PhIO. However, compared with those in electron transfer, improvements in hydrogen abstraction and electron transfer are minor. The existence of the interaction between Lewis acid and the manganese(IV) species was evidenced by the blue shift of the characteristic absorbance of the manganese(IV) species from 554 to 537 nm and by converting its EPR signal at g = 2.01 into a hyperfine 6-line signal upon adding Al(3+) (I = 5/2). Cyclic voltammograms of the manganese(IV) complexes reveal that adding Lewis acid would substantially shift its potential to the positive direction, thus enhancing its oxidizing capability.